1. Field of the Invention
The present invention relates to a Global Positioning System (GPS) and a Global Positioning method for precisely determining a location by receiving GPS signals from satellites.
2. Description of Related Art
Many satellites orbit the earth, and continuously transmit radio waves at a carrier frequency of 1575.42 GHz. The radio waves are phase modulated by pseudo-random sequences, and a unique pattern is assigned to each satellite so that the different radio waves can be easily identified. As a typical pseudo-random sequence, is known a regularly modulated code pattern called a C/A code (clear and acquisition code) available to the public. Furthermore, the radio waves carry navigation data necessary for users to perform positioning, such as satellite orbit information, satellite correction data, correction coefficients of the ionosphere, etc. The navigation data are transmitted by means of polarity inversions in the C/A code sequence.
FIG. 13 is a diagram showing the C/A code sequence. As shown in FIG. 13, the C/A code sequence is a regularly arranged code sequence with its data consisting of 20 PN frames, each of which consists of 1023 bits one millisecond long. Thus, the navigation data is a 50 bit per second (BPS) signal consisting of 1000 PN frames per second. The polarity of the C/A code sequence is reversed in accordance with the polarity of the bits of the navigation data.
FIG. 14 is a block diagram showing a configuration of a conventional Global Positioning System disclosed in U.S. Pat. No. 5,663,734. In this figure, the reference numeral 101 designates a base station having a GPS receiving antenna 102 and a transmitting and receiving antenna 103. The reference numeral 104 designates a remote unit.
The remote unit 104 comprises an RF (radio frequency) to IF (intermediate frequency) converter 106 with a GPS receiving antenna 105; an A/D converter 107 for converting the analog signal from the converter 106 to a digital signal; a memory (digital snapshot memory) 108 for recording the output of the A/D converter 107; and a general purpose programmable digital signal processor 109 (called DSP from now on) for processing the signal fed from the memory 108.
The remote unit 104 further comprises a program EPROM 110 connected to the DSP 109, a frequency synthesizer 111, a power regulator 112, a write address circuit 113, a microprocessor 114, a RAM (memory) 115, an EEPROM 116, and a modem 118 which has a transmitting and receiving antenna 117, and is connected to the microprocessor 114.
Next, the operation of the conventional GPS will be described.
The base station 101 commands the remote unit 104 to perform a measurement via a message transmitted over a data communication link 119. The base station 101 also sends within this message Doppler information for the satellites in view, which is a form of satellite data information. This Doppler information typically is in the format of frequency information, and the message will specify an identification of the particular satellites in view. This message is received by the modem 118 in the remote unit 104, and is stored in the memory 108 connected to the microprocessor 114.
The microprocessor 114 handles data information transfer between the modem 118 and the DSP 109 and write address circuit 113, and controls the power management functions in the remote unit 104.
Once the remote unit 104 receives a command (e.g., from the base station 101) for GPS processing together with the Doppler information, the microprocessor 114 activates the RF to IF converter 106, A/D converter 107 and memory 108 via the power regulator 112 and controlled power lines 120a-120d, thereby providing full power to these components. This causes the signal from the GPS satellite which is received by the antenna 105 to be down-converted to an IF frequency, followed by conversion to digital data.
A contiguous set of such data, typically corresponding to a duration of 100 milliseconds to one second (or even longer), is stored in the memory 108.
Pseudo range calculation is executed by the DSP 109 that uses a fast Fourier transform (FFT) algorithm, which permits very rapid computation of the pseudo ranges by performing quickly a large number of correlation operations between a locally generated reference and the received signals. The fast Fourier transform algorithm permits a simultaneous and parallel search of all positions, thus speeding up the required computation process.
Once the DSP 109 completes its computation of the pseudo ranges for each of the in view satellites, it transmits this information to the microprocessor 114 through an interconnect bus 122.
Then, the microprocessor 114 utilizes the modem 118 to transmit the pseudo range data over the data link 119 to the base station 101 for final position computation.
In addition to the pseudo data, a time lag may simultaneously be transmitted to the base station 101 that indicates the elapsed time from the initial data collection in the memory 108 to the time of transmission over the data link 119. This time lag improves the capability of the base station 101 to perform position calculation, since it allows the computation of the GPS satellite positions at the time of data collection.
The modem 118 utilizes a separate transmitting and receiving antenna 117 to transmit and receive messages over the data link 119. The modem 118 includes a communication receiver and a communication transmitter, which are alternately connected to the transmitting and receiving antenna 117. Similarly, the base station 101 may use a separate antenna 103 to transmit and receive data link messages, thus allowing continuous reception of GPS signals via the GPS receiving antenna 102 at the base station 101.
It is expected that the position calculations in the DSP 109 will require less than a few seconds of time, depending upon the amount of the data stored the memory 108 and the speed of the DSP 109 or several DSPs.
As described above, the memory 108 captures a record corresponding to a relatively long period of time. The efficient processing of this large block of data using fast convolution methods contributes to the ability to process signals at low received levels such as when reception is poor due to partial blockage from buildings, trees etc.
All pseudo ranges for visible GPS satellites are computed using the same buffered data. This provides improved performance relative to continuous tracking GPS receivers in situations such as urban blockage conditions in which the signal amplitude is rapidly changing.
The signal processing carried out by the DSP 109 will now be described with reference to FIG. 13. The objective of the processing is to determine the timing of the received waveform with respect to a locally generated waveform. Furthermore, in order to achieve high sensitivity, a very long portion of such a waveform, typically 100 milliseconds to one second, is processed.
The received GPS signal (C/A mode) is constructed from a high rate (1 MHz) repetitive pseudo random (PN) pattern (PN frame) of 1023 symbols, and successive PN frames are added to one another. For example, there are 1000 PN frames over a period of one second. The first such frame is coherently added to the next frame, the result added to the third frame, followed by the additions as shown in FIGS. 15(A)-15(E). The result is a signal having a duration of one PN frame (=1023 chips). The phase of this sequence is compared to a local reference sequence to determine the relative timing between the two, thus establishing the pseudo range.
With the foregoing configuration, the conventional Global Positioning System carries out preprocessing operation which precedes the correlation calculations, and which is called xe2x80x9cpreliminary integration of the received GPS signalxe2x80x9d to implement high sensitivity. In this process, the preliminary integration is carried out for 5-10 PN frames to avoid reduction in the integrals due to polarity inversions in the navigation data.
The C/A code sequence in the GPS received signal can change its phases, that is, have polarity inversions at the transitions of the bits of the navigation. Therefore, the signal components (chips) may cancel out each other in the integration (cumulative summing) process because of the polarity inversions at the bits of the navigation data in the C/A code sequence, hindering sufficient improvement in the sensitivity (S/N ratio).
In other words, the conventional system does not detect the polarity inversions in the navigation data.
This limits the theoretical number of data to be integrated, and hence presents a problem of providing only insufficient improvement in the sensitivity (S/N ratio).
In addition, every time it determines its position (called xe2x80x9cpositioningxe2x80x9d from now on), the remote unit functioning as a terminal collects Doppler information from the base station, calculates pseudo ranges to the visible satellites, and determines its position by transmitting the distance information to the server. Thus, the positioning always requires communication with the server, offering a problem of entailing communication cost.
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a highly sensitive Global Positioning System and Global Positioning method that can reduce the communication cost by limiting communications with a server only to a case where the receiving sensitivity is insufficient, and that can achieve stable reception inside buildings or in the blockage therefrom.
According to a first aspect of the present invention, there is provided a Global Positioning System including an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, and a GPS terminal that is connected to the external system through a communication medium and receives the GPS signal from the satellite, wherein each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the GPS terminal comprising: a frequency converter for converting a frequency of the GPS signal received by the GPS terminal; an A/D converter for converting the GPS signal passing through the frequency conversion into corresponding GPS data; a memory for storing the GPS data for a . predetermined time interval; a Doppler correction section for performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; means for sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; means for calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; means for multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; means for summing up the plurality of products at respective chip positions; and means for computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal, and for calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum.
Here, the Global Positioning System may further comprise means for shifting divisions of the data blocks if the correlation peak is less than a prescribed level, and for supplying new data blocks to the means for calculating a cumulative sum.
The means for shifting divisions of the data blocks may further shift the divisions to maximize the correlation, and the means for computing correlation may determine the pseudo range from the correlation peak position.
The Global Positioning System may further comprise a received electric field detector for detecting intensity of a received electric field, wherein the means for sequentially dividing the GPS data may divide, when the received electric field is greater than a predetermined level, the GPS data passing through the Doppler correction into the data blocks in accordance with the navigation data obtained from the GPS signal received by the GPS terminal, and may divide, when the received electric field is less than the predetermined level, the GPS data in accordance with the navigation data supplied from the external system.
The Global Positioning System may further comprise a position determining section for determining a position of the GPS terminal from the pseudo ranges and the navigation data which is extracted from the GPS signal received by the GPS terminal when the received electric field is greater than a prescribed level, and which is supplied from the external system when the received electric field is less than the prescribed level.
The GPS terminal may further comprise a position determining section for determining a position of the GPS terminal from the pseudo ranges obtained by the GPS terminal and the navigation data received by the external system.
The means for shifting divisions of the data blocks may shift the divisions of the data blocks by a predetermined amount if the correlation peak value is less than a prescribed level to enable detection of the correlation peak position.
The means for shifting divisions of the data blocks may shift the divisions of the data blocks roughly at a first step, and then slightly at a second step to converge to the correlation peak value.
The Doppler correction section may carry out the Doppler correction of the C/A code sequence in the GPS signal received by the GPS terminal using the Doppler information obtained from the navigation data supplied by one of the GPS terminal and the external system.
The Doppler correction section may carry out the Doppler correction of the C/A code sequence by performing the Doppler correction on the GPS signal received by the GPS terminal.
The received electric field detector may change a correlation calculation interval in response to the electric field level detected.
The received electric field detector may change a summation interval in response to the electric field level detected.
According to a second aspect of the present invention, there is provided a GPS terminal connected through a communication medium to an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, wherein the GPS terminal receives the GPS signal from the satellite, and each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the GPS terminal comprising: a frequency converter for converting a frequency of the GPS signal received by the GPS terminal; an A/D converter for converting the GPS signal passing through the frequency conversion into corresponding GPS data; a memory for storing the GPS data for a predetermined time interval; a Doppler correction section for performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; means for sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; means for calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; means for multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; means for summing up the plurality of products at respective chip positions; and means for computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal, and for calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum.
According to a third aspect of the present invention, there is provided a Global Positioning method in a system including an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, and a GPS terminal that is connected to the external system through a communication medium and receives the GPS signal from the satellite, wherein each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the Global Positioning method comprising the steps of: converting a frequency of the GPS signal received by the GPS terminal; carrying out A/D conversion of the GPS signal passing through the frequency conversion into corresponding GPS data; storing the GPS data for a predetermined time interval; performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; summing up the plurality of products at respective chip positions; computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal; and calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum.